The disposable automatic blood lancet, as the main development stream of the current blood lancet, has been improved and developed constantly for many years. The Chinese patent once announced on Jun. 22, 2005 to authorize a utility model patent titled “Casing Self-Locking Disposable Safe Automatic Blood Lancet” with a patent number of 200420026368.7 and an announcement number of CN2705119Y (with this patent enjoying the right of priority, an American patent is applied for afterwards with a patent number of U.S. Pat. No. 7,322,997B2). In this patent, a casing forms an ejection cavity with a pinhole, a lancet core is arranged inside the ejection cavity, and an ejection structure is formed including a spring and a bayonet ejection structure. A button is arranged on the casing; and a self-locking structure is formed by a barb on the button engaged with a self-locking opening on the casing. When the button is pushed, the bayonet ejection structure will be triggered to make the lancet core disengaged from the casing, and then the spring will push the lancet core to eject. After the ejection, because of the movement of the button, the barb moves forward beyond the self-locking opening; therefore, the barb is locked at the self-locking opening in the rebounding process, and cannot go back to the pre-ejection state, making the bayonet ejection structure fail and unreusable. This design is compact in structure and skillful in concept, and particularly it can make the blood lancet enter the self-locking state after ejection by locking the button through the casing engaged with the button, thus attaining the purpose of making the blood lancet unreusable. However, it can be seen from specification or drawings of this patent that the following two operation actions have to be done before using this blood lancet: First the protective bar has to be pushed to make the lancet core compress the spring and blocked at the bayonet ejection structure; and then the protective bar has to be twisted off to make the lancet core enter the ejection state. Considering facilitating application by the medical care personnel, the fewer the operation actions the better. As for this patented product, the protective bar can be assembled into a post-push state during assembly in the manufacturing companies so as to reduce number of the operation actions. However, this will then result in a hidden trouble with the blood lancet, since even if the protective bar is not twisted off, the bayonet ejection structure may be triggered through the button during transportation and before application. This is also the first problem with this patented product. The second problem is that when the medical care personnel use this blood lancet for the first time, they may not read the operation instruction in detail, and may twist off the protective bar during application without pushing the protective bar first, which will also make the blood lancet fail to be ejected due to the blood lancet not being blocked into the bayonet ejection structure. Therefore, the purpose of the present invention is to resolve these problems.